In recent years, various types of color liquid-crystal-display units have been put in practical use. As one of these color liquid-crystal-display units, a color liquid crystal projector is used. The color liquid crystal projector has many types, and each of them incorporates three liquid crystal panels. More specifically, a red-color.(R) optical image, a green-color (G) optical blue-color (B) optical image are separately displayed by the three liquid crystal panels. The three optical images having different colors are combined with a optical system to project on a screen as a full-color image.
Such a three-panel-type color liquid crystal projector requires not only three expensive liquid crystal panels but also a large number of optical components to separate a lightwave or combine three lightwaves having different colors. In other words, the three-panel-type color liquid crystal projector is not only high-cost but also difficult to be miniaturized.
On the other hand, a single-panel-type color liquid crystal projector enables cost reduction and miniaturization. An example of a single-panel-type color liquid crystal projector put into a practical use is a rear-projection television (HV-D50LA1) produced by Victor Company of Japan, Limited.
FIG. 9 is a schematic cross section showing the basic optical principle employed in the single-panel-type color liquid crystal projector incorporated into HV-D50LA1 of Victor Company of Japan, Limited. The color liquid crystal projector has a hologram color filter, which is shown as a hologram film 2 formed on the upper surface of a glass substrate 1. A liquid crystal layer 3 is provided on the back face of the glass substrate 1. A reflection-type electrode layer 4 is provided on the back face of the liquid crystal layer 3. The reflection-type electrode layer 4 has reflection-type pixel electrodes each for reflecting an R, G, or B lightwave. A combination of R, G, and B reflection-type electrodes composes a single pixel. In FIG. 9, a plurality of R electrodes are arranged in a direction orthogonal to the drawing. Similarly, a plurality of G electrodes and a plurality of B electrodes are both arranged in a direction orthogonal to the drawing. In addition, a transparent electrode (not shown) is provided between the glass substrate 1 and the liquid crystal layer 3 as a counter electrode to the R, G, and B electrodes.
In the color liquid crystal projector shown in FIG. 9, a white lightwave, W, emitted from a light source (not shown) enters the hologram film 2 at a specified incident angle. The hologram film 2 by its diffraction function separates the white lightwave, W, into a red-color (R) lightwave, green-color (G) lightwave, and blue-color (B) lightwave according to their wavelengths (separation of a lightwave into its spectral components). The hologram film 2 also has a function as a microlens array for gathering the three types of lightwaves onto the corresponding R, G, and B electrodes. In this case, the red-color (R), green-color (G), and blue-color (B) lightwaves reflected from the R, G, and B electrodes, respectively, are displaced from one another according to the diffraction condition of the hologram film 2. The displaced three types of lightwaves pass through the hologram film 2 to be projected onto a screen by a projection lens (not shown).
FIG. 10 is a schematic plan view showing an example of a hologram (diffraction grating) having both a light-separating function and a microlens array function. The diffraction grating has a grating pattern formed on a glass substrate 11. The grating pattern has a plurality of belt-shaped regions 12 parallel to one another. The belt-shaped regions 12 may be formed with metallic chromium (Cr) films, for example. Of course, the Cr films 12 do not transmit a lightwave, and a lightwave passes only through the spacing between the multiple belt-shaped Cr films 12.
In other words, the multiple parallel belt-shaped Cr-film regions 12 act as a diffraction grating, and the lightwave is diffracted in a direction orthogonal to the longitudinal direction of the belt-shaped Cr-films 12. At this moment, as is well known, because the diffraction angle has wavelength dependence, the R, G, and B lightwaves are diffracted with diffraction angles different with one another. Thus, the white-color lightwave, W, can be color-separated.
Furthermore, the diffraction grating shown in FIG. 10 has a special feature in that the width and spacing of the belt-shaped Cr-film regions 12 are periodically varied. This arrangement is performed to give the diffraction grating a microlens array function. More specifically, when the wavelength is the same, as is well known, as the spacing of the diffraction grating decreases, the diffraction angle increases. Therefore, by gradually varying the spacing of the diffraction grating, the lens function can be produced.
In addition, in the diffraction grating shown in FIG. 10, as described above, the lightwave is diffracted only in a direction orthogonal to the longitudinal direction of the belt-shaped Cr-films 12. Therefore, the lens function is produced only in that direction. As a result, the diffraction grating acts as a columnar lens having a linear focus. However, if required, by utilizing a diffraction grating similar to the well-known Fresnel's zone plate, it is possible to produce a function of a circular or square lens having a point focus, as a matter of course.
The diffraction grating shown in FIG. 10 acts as if it has a plurality of columnar microlenses parallel to one another. The region indicated by an arrow 13 acts as a single columnar microlens. In the columnar microlens region 13, as the position moves from the right to the left, the width and spacing of the belt-shaped Cr films 12 are decreased. In other words, in the diffraction grating shown in FIG. 10, the width and spacing of the belt-shaped Cr films 12 are periodically varied for every columnar microlens region 13.
When the diffraction grating as shown in FIG. 10 is used without modification in place of the hologram film 2 in the color liquid crystal projector shown in FIG. 9, because the belt-shaped Cr-films 12 do not transmit a lightwave, the utilization efficiency of the white-color lightwave, W, from the light source is decreased. In addition, in the diffraction grating shown in FIG. 10, the pitch of the belt-shaped Cr-film regions 12 is remarkably small. For example, at the center portion of the region 13, the pitch is at most 0.5 μm or so. Consequently, the diffraction grating as shown in FIG. 10 must be formed by using the electron-beam drawing, which is unsuitable for the industrial mass production.
To solve this problem, in the color liquid crystal projector shown in FIG. 9, light is applied to a photopolymer film on the glass substrate through a master diffraction grating. Then, the light-irradiated photopolymer film is heat-treated to produce the hologram film 2. At this moment, as the intensity of the light applied to a region is increased, the refractive index “n” of the region is increased. In other words, the hologram film 2 made of a photopolymer has a modulated refractive index, “n,” and acts as a refractive-index-modulated-type diffraction grating.
FIG. 11 is a schematic cross section illustrating a color liquid-crystal-display unit disclosed in the patent literature 1, which is the published Japanese patent application Tokukaihei 10-96807. The color liquid-crystal-display unit has a well-known light-transmitting-type liquid crystal panel 40. The liquid crystal panel 40 has a liquid crystal displaying layer 41 and a black matrix 42. The liquid crystal displaying layer 41 has a plurality of pixels, and each pixel has a single combination of a red-color-displaying region R, a green-color-displaying region G, and a blue-color-displaying region B. The boundaries between the individual color-displaying regions are covered with the black matrix 42.
A hologram color filter 50 is placed at the rear-face side of the liquid crystal panel 40. The hologram color filter 50 has a hologram plate 51 and an array of a plurality of microlenses 52. The individual microlenses 52 are arranged in an array with a period corresponding to that of the pixels in the liquid crystal panel 40. The hologram plate 51 is formed with a silica-glass plate that has parallel and uniform grooves and that functions as a diffraction grating.
In the color liquid-crystal-display unit shown in FIG. 11, when a back lightwave 60 is introduced into the hologram color filter 50, the lightwave 60 is diffracted at different angles according to the wavelengths to perform wavelength separation. Then, a red-color lightwave 61, a green-color lightwave 62, and a blue-color lightwave 63 emerge from the exit side of the hologram plate 51. The microlens 52 placed next to the hologram plate 51 gathers these wavelength-separated lightwaves on their focal planes by separating the lightwaves according to their wavelengths. More specifically, the color filter 50 is arranged such that the red-color lightwave 61 is diffracted to gather at the red-color-displaying region, R, in the pixel, the green-color lightwave 62 at the green-color-displaying region, G, and the blue-color lightwave 63 at the blue-color-displaying region, B. This arrangement enables the individual lightwaves each having a specific color component to pass through the liquid crystal cells almost without being attenuated at the black matrix 42. Thus, the color display of the individual liquid crystal cells can be performed.
In such a color liquid-crystal-display unit, as the hologram plate 51, a light-transmitting-type hologram plate is used that has no light-gathering property and has less wavelength dependence of the diffraction efficiency. Consequently, it is not necessary to align the hologram plate 51 with the arranging period of the microlenses 52. Furthermore, unlike the case where a single microlens is placed corresponding to each of the color-displaying regions, a single microlens 52 is placed corresponding to each pixel, increasing the arranging period threefold. As a result, the microlens array becomes easy to produce and arrange.
FIG. 12 is a schematic cross section illustrating a hologram color filter disclosed in the non-patent literature 1, which is ITE Technical Report Vol. 20, 1996, pp. 69-72. The hologram color filter has two hologram films 71 and 72 in order to improve the intensity balance between the red-color, green-color, and blue-color lightwaves.
In general, in a hologram film, there exists a wavelength of a lightwave that is most easily diffracted by the hologram film. More specifically, a hologram film has the highest diffraction efficiency for a lightwave having a specific wavelength. As the wavelength difference from the specific wavelength increases, the diffraction efficiency tends to decrease. In particular, in the case where the refractive-index difference, Δn, is small in a refractive-index-modulated-type hologram film, this wavelength dependence of the diffraction efficiency tends to be remarkable. For example, as in a photopolymer hologram film, when the refractive-index difference, Δn, is as small as at most 0.04, it is difficult to obtain a hologram film that has a small wavelength dependence of the diffraction efficiency.
Consequently, in the case where a hologram film wavelength-separates a white-color lightwave into a red-color lightwave, green-color lightwave, and blue-color lightwave, the hologram film is designed such that the highest diffraction efficiency can be achieved for the green-color lightwave, which lies in an intermediate wavelength range among the red-color, green-color, and blue-color lightwaves. A hologram film designed by the above-described principle has a lower diffraction efficiency for the red-color and blue-color lightwaves than for the green-color lightwave. Consequently, the red-color and blue-color lightwaves that are wavelength-separated by the hologram film have a lower intensity than that of the green-color lightwave. As a result, even when the wavelength-separated red-color, green-color, and blue-color lightwaves are recombined with the intention of achieving a white-color lightwave, the recombined lightwave tends to be a greenish white-color lightwave.
In addition, a halide lamp and a superhigh-pressure mercury lamp to be used as a back light for a color liquid-crystal-display unit include an intense emission line in a wavelength range of the green-color lightwave. Therefore, when the lightwave from a halide lamp or a superhigh-pressure mercury lamp is wavelength-separated by a hologram film designed to have the highest diffraction efficiency for the green-color lightwave, the intensity of the green-color lightwave tends to be most remarkable among the wavelength-separated red-color, green-color, and blue-color lightwaves.
The hologram color filter shown in FIG. 12 has two hologram films 71 and 72 in order to decrease the non-uniformity in the above-described wavelength-dependent diffraction efficiency so that the color balance of the color liquid-crystal-display unit can be improved. The first hologram film 71 has a diffraction efficiency of η1 for the lightwave having a specific wavelength of λ, and the second hologram film 72 has a diffraction efficiency of η2 or the lightwave having the same specific wavelength of λ. Here, the diffraction efficiency is assumed to take a value of 1 when the entire incident lightwave is diffracted and a value of 0 when the entire incident lightwave is transmitted without being diffracted.
When the incident lightwave of the intensity 1 having a specific wavelength of λ passes through the first hologram film 71, the intensity ratio of the transmitted lightwave to the diffracted lightwave is (1−η1):η1. When the transmitted lightwave having passed through the first hologram film 71 passes through the second hologram film 72, the intensity ratio of the transmitted lightwave (parallel to the direction of the original incident lightwave) to the diffracted lightwave (parallel to the direction of the diffraction by the first hologram film 71) is (1−η1)(1−η2):η2(1−η1). When the diffracted lightwave having passed through the first hologram film 71 passes through the second hologram film 72, the intensity ratio of the diffracted lightwave (parallel to the direction of the original incident lightwave) to the transmitted lightwave (parallel to the direction of the diffraction by the first hologram film 71) is η1η2:η1(1−η2). Consequently, the lightwave having passed through the two hologram films 71 and 72 and being in the direction of the diffraction has an intensity of η2(1−η1)+η1(1−η2)=η1+η2−2η1η2.
FIG. 13 shows an example of the result of a computer simulation on the hologram color filter having two hologram films as shown in FIG. 12. In the graph in FIG. 13, the horizontal axis represents the wavelength (nm) of the lightwave, and the vertical axis the diffraction efficiency of the hologram film.
A curve “a” shows an example of the diffraction efficiency of a hologram color filter made of a single hologram film. The single hologram film “a” is designed such that the highest diffraction efficiency can be achieved for the green-color lightwave, which has an intermediate wavelength between the red-color and blue-color lightwaves. Consequently, after a white-color lightwave is wavelength-separated by the hologram film “a,” the red-color and blue-color lightwaves tend to have a lower intensity than that of the green-color lightwave.
On the other hand, the hologram film “b” is designed such that the highest diffraction efficiency can be achieved for the red-color lightwave, and the hologram film “c” is designed such that the highest diffraction efficiency can be achieved for the blue-color lightwave. As a result, the hologram color filter having the two hologram films “b” and “c” has a combined diffraction efficiency as shown by a curve “d.” In other words, the hologram color filter “d” has a higher diffraction efficiency for the red-color and blue-color lightwaves than for the green-color lightwave. Because it has two peaks of diffraction efficiency, it is sometimes called a two-peak hologram color filter.
FIG. 14 is a schematic cross section illustrating a color liquid crystal projector disclosed in the patent literature 2, which is the published Japanese patent application Tokukai 2000-235179. This color liquid-crystal-display unit comprises a white-color light source 81, three dichroic mirrors 82, a glass substrate 83, a hologram lens layer 84 made of a photopolymer, a thin-plate glass layer 85, a transparent electrode 86, a liquid crystal layer 87, a pixel electrode 88, an active-matrix-driving circuit 89, and a projecting lens 90.
In the color liquid crystal projector shown in FIG. 14, a white-color lightwave emitted from the white-color light source 81 is wavelength-separated into three primary color lightwaves (R, G, and B) by the three dichroic mirrors 82. The wavelength-separated R, G, and B lightwaves are projected onto the hologram lens layer 84 with incident angles different from one another such that each of them can be gathered at the highest diffraction efficiency.
Patent literature 1: the published Japanese patent application Tokukaihei 10-96807.
Patent literature 2: the published Japanese patent application Tokukai 2000-235179.
Non-patent literature 1: ITE Technical Report Vol. 20, 1996, pp. 69-72.